Doc: Fix botched attempt of documenting the 'relocatable' feature

We don't have a -relocatable configure switch. Move the documentation to
the right place (configure -list-features).

The correct configure switches to turn the feature on/off are
-feature-relocatable and -no-feature-relocatable.

Task-number: QTBUG-80928
Change-Id: I96df0f44f12b2efe5a64132e9006d4b73de41255
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Tasuku Suzuki <tasuku.suzuki@kdab.com>
Reviewed-by: Yuhang Zhao <2546789017@qq.com>

config_help.txt

@@ -163,8 +163,6 @@ Build options:
   -reduce-exports ...... Reduce amount of exported symbols [auto]
   -reduce-relocations .. Reduce amount of relocations [auto] (Unix only)
 
-  -relocatable ......... Enable the Qt installation to be relocated [auto]
-
   -plugin-manifests .... Embed manifests into plugins [no] (Windows only)
   -static-runtime ...... With -static, use static runtime [no] (Windows only)
 

configure.json

@@ -1380,6 +1380,7 @@
         },
         "relocatable": {
             "label": "Relocatable",
+            "purpose": "Enable the Qt installation to be relocated.",
             "autoDetect": "features.shared",
             "condition": "features.dlopen || config.win32 || !features.shared",
             "output": [ "privateFeature" ]